Disposed in an exhaust passage of an external combustion engine are exhaust system components such as sensors for detecting data (for example, exhaust air/fuel ratio and exhaust gas temperature) relating primarily to the exhaust passage and also to an exhaust purification catalyst and exhaust gas, and those components are exposed to the exhaust gas flowing in the exhaust passage. The exhaust gas temperature changes according to the operation state of the internal combustion engine, and where the exhaust system components are exposed to the exhaust gas at a comparatively high temperature, the components can be damaged by the exhaust heat. A technique for preventing such damage caused by exhaust heat is disclosed, for example, in Patent Literature 1. With such a technique, a thermal insulation material is provided in the exhaust passage between an exhaust manifold and an exhaust purification catalyst at the inner circumferential surface of the passage where the exhaust temperature peaks, thereby protecting the exhaust passage. In this case, heat radiation from the exhaust passage outside the location where the thermal insulation material is disposed is maintained thereby still ensuring certain cooling of the exhaust gas and protecting the exhaust system components.
With another method for cooling exhaust gas, the amount of fuel contained in the exhaust gas is increased and the latent heat of vaporization thereof is used, but increasing the amount of fuel in exhaust gas is undesirable from the standpoint of emission since the concentration of carbon monoxide is easily increased. Mounting a cooling adapter of a water cooling system on an exhaust pipe is also a conventional technique as a technique for cooling the exhaust gas. Where such a cooling adapter is used, thermal load on exhaust system components can be effectively reduced, but since the exhaust gas is cooled even when the exhaust gas temperature is comparatively low, for example, in the cold start of the internal combustion engine, the exhaust purification catalyst is difficult to warm up rapidly. Accordingly, a technique has been disclosed by which a non-contact region is formed in a flange portion so as to reduce the contact surface area between the cooling adapter and the exhaust gas manifold (see, for example, Patent Literature 2).
Further, a cooling liquid flow channel for cooling an internal combustion engine has been formed in the cylinder head of the external combustion engine. A problem that can be associated with such a configuration is that the heat of exhaust gas is also taken away through the exhaust passage by the cooling liquid flowing in the flow channel, thereby decreasing the exhaust gas temperature. To resolve this problem, Patent Literature 3 discloses a technique relating to a mounting structure of a double-wall tubular exhaust manifold on a cylinder head side and suggests the structure of a mounting flange that inhibits the transfer of heat from the exhaust manifold side to the cylinder head.
Patent Literature 1: Japanese Patent Application Publication No. 2006-2604
Patent Literature 2: Japanese Patent Application Publication No. 2011-169311
Patent Literature 3: Japanese Patent Application Publication No. H9-242537
Patent Literature 4: Japanese Patent Application Publication No. 2002-115601
Patent Literature 5: Japanese Patent Application Publication No. 2004-156547